WRT54GX4 & VPN

I have a GX4 and am trying to get into my company network using a Symantec VPN client (v8 ). I can connect to the VPN without a problem. However, once I get there I can't do anything (can't login, can't ping any of the servers, etc, etc).

Others at the office VPN so I know it's not a problem on the server side. IT is trying to troubleshoot, but have told me that it could be a router problem on my side.

I'm using firmware v.15 firmware, have the SPI firewall disabled and all of the VPN passthrough checkboxes enabled. The only thing we've been able to find on the Symantec side is that they recommend UDP port 500 opened bidirectionally (which I have no idea how to do in this firmware, or even how to check whether it is open or not).